Current interrupter for electrochemical cells

ABSTRACT

A current interrupt mechanism for electrochemical cells is disclosed. A thermally activated current interrupt mechanism is integrated into an end cap assembly for an electrochemical cell. The thermally responsive mechanism preferably includes a free floating bimetallic disk which deforms when exposed to elevated temperature causing a break in an electrical pathway within the end cap assembly. This prevents current from flowing through the cell and effectively shuts down an operating cell. Alternatively, the thermally responsive mechanism may include a meltable mass of material which melts when exposed to elevated temperature to break an electrical pathway within the end cap assembly. The end cap assembly may also include integrated therein a pressure responsive current interrupt mechanism. If the internal gas pressure within an operating cell exceeds a predetermined value, the pressure responsive mechanism activates to sever an electrical pathway within the end cap assembly to prevent current from passing through the cell. The pressure responsive mechanism may include a diaphragm which ruptures when there is extreme gas pressure buildup. Gas is allowed to escape from the cell interior to the external environment through a series of vent apertures within the end cap assembly.

This application claims benefit of Provisional application Ser. No.60/015,153 filed Apr. 10, 1996.

FIELD OF THE INVENTION

This invention relates to a thermally responsive current interrupter foran electrochemical cell, which safely prevents current flow through thecell upon an excessive increase in the temperature thereof. Theinvention also relates to a pressure responsive current interrupter fora cell, which safely shuts down the cell upon excessive gas pressurebuildup therein.

BACKGROUND OF THE INVENTION

Electrochemical cells, especially high energy density cells such asthose in which lithium is an active material, are subject to leakage orrupture which, in turn, can cause damage to the device which is poweredby the cell or to the surrounding environment. In the case ofrechargeable cells, the rise in internal temperature of the cell canresult from overcharging. Undesirable temperature increases are oftenaccompanied by a corresponding increase in internal gas pressure. Thisis likely to occur in the event of an external short circuit condition.It is desirable that safety devices accompany the cell without undulyincreasing the cost, size or mass of the cell.

Such cells, particularly rechargeable cells utilizing lithium as anactive material, are subject to leakage or rupture caused by a rise ininternal temperature of the cell which often is accompanied by acorresponding increase in pressure. This is likely to be-caused byabusive conditions, such as overcharging or by a short circuitcondition. It is also important that these cells be hermetically sealedto prevent the egress of electrolyte solvent and the ingress of moisturefrom the exterior environment.

As set forth above, as such a cell is charged, self-heating occurs.Charging at too rapid a rate or overcharging can lead to an increase inthe temperature. When the temperature exceeds a certain point, whichvaries depending upon the chemistry and structure of the cell, anundesirable and uncontrollable thermal runaway condition begins. Inaddition, because of the overheating, internal pressure builds up, andelectrolyte may suddenly be expelled from the cell. It is preferable toinitiate controlled venting before that takes place.

Conventional cell designs employ an end cap fitting which is insertedinto an open ended cylindrical casing after the cell anode and cathodeactive material and appropriate separator material and electrolyte havebeen inserted into the cylindrical case. The end cap is in electricalcontact with one of the anode or cathode material and the exposedportion of the end cap forms one of the cell terminals. A portion of thecell casing forms the other terminal. The prior art discloses meansresponsive to over pressure conditions which have been integrated intothe cell end cap fitting.

SUMMARY OF THE INVENTION

The present invention has one or several current interrupt mechanismsintegrated within a single end cap assembly which may be appliedadvantageously to primary or secondary (rechargeable) cells, forexample, by inserting the end cap assembly into the open end of a casingfor the cell. The end cap assembly of the invention has particularapplication to rechargeable cells, for example lithium-ion, nickel metalhydride, nickel cadmium or other rechargeable cells, to overcome thedanger of the cell overheating and pressure building up in the cellduring exposure to high temperatures, excessive or improper charging, orshorting of the cell.

In one aspect the invention is directed to an end cap assembly for anelectrochemical cell wherein the end cap assembly has integrated thereina thermally responsive current interrupt mechanism which activates tointerrupt and prevent current from flowing through the cell when thecell interior overheats to exceed a predetermined temperature. The endcap assembly has an exposed end cap plate which functions as a terminalof the cell. When the assembly is applied to a cell and the cell is innormal operation the end cap plate is in electrical communication with acell electrode (anode or cathode). The thermally activated currentinterrupt mechanism integrated within the end cap assembly may comprisea bimetallic member that deflects when exposed to temperature above apredetermined value. The deflection of the bimetallic member pushesagainst a movable metal member to sever electrical connection between anelectrode of the cell and the end cap terminal plate thus preventingcurrent from flowing through the cell. Alternatively, in another aspectof the invention a thermally responsive pellet may be used instead ofthe bimetallic member. If the temperature of the cell exceeds apredetermined value, the thermal pellet melts causing a metallic membersupported thereon to deflect sufficiently to sever the electricalpathway between an electrode of the cell and the end cap terminal plate.A rupturable plate or membrane may be integrated into the end capassembly along with the thermally responsive current interruptmechanism. When pressure within the cell builds up to exceed apredetermined value the plate or membrane ruptures allowing gas from theinterior of the cell to escape to the external environment.

In another aspect the invention is directed to an end cap assembly forcells, particularly rechargeable cells, wherein the end cap hasintegrated therein two current interrupt mechanisms one being thermallyresponsive and the other being pressure responsive. The thermallyresponsive current interrupt mechanism may preferably employ abimetallic member or thermally responsive meltable pellet whichactivates to interrupt and prevent current flow through the cell whenthe cell interior overheats to exceed a predetermined temperature. Thepressure responsive current interrupt mechanism activates to interruptcurrent flow when gas pressure in the cell builds up to exceed apredetermined value. In such case, the pressure interrupt mechanism maycause a metal diaphragm within the end cap assembly to deflect therebysevering the electrical connection between the cell end cap terminalplate and a cell electrode, thereby preventing current from flowingthrough the cell. In the case of extreme gas pressure buildup the metaldiaphragm also ruptures allowing gas to be channeled into interiorchambers within the end cap assembly and out to the external environmentthrough a series of vent holes.

Another aspect of the invention is directed to a sealing mechanism forthe end cap assembly of the invention. The sealing mechanism preventsleakage of electrolyte, liquid or gas from the end cap interior to theexternal environment and prevents ingress of moisture into the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will be better appreciated with referenceto the drawings in which:

FIGS. 1, 2 and 3 are vertical cross-sectional views taken through sightlines 1--1 of the end cap assembly of FIG. 6.

FIG. 1 shows the thermally activated current interrupt mechanism andpressure activated current interrupt mechanism in circuit connectedmode.

FIG. 2 shows the thermally activated current interrupt mechanism incircuit interrupted mode.

FIG. 3 shows the pressure activated current interrupt mechanism inpressure activated, circuit interrupted mode.

FIG. 4 is a vertical cross-sectional view of another embodiment of anend cap assembly having pressure activated current interrupt mechanismand thermally activated current interrupt mechanism integrated thereinin which a heat sensitive member softens to release a resilient memberto open the circuit.

FIG. 5 is an exploded perspective view of the components of end capassembly of the invention shown in the embodiment of FIG. 1.

FIG. 6 is a perspective view of the bottom of the end cap assemblyshowing the pressure resistant plate and vent apertures therethrough.

FIG. 7 is a perspective view showing the end cap assembly of theinvention being inserted into the open end of a cylindrical casing of acell.

FIG. 8 is a perspective view showing a completed cell with the end capassembly of the invention inserted into the open end of a cylindricalcasing of a cell with end cap plate of said assembly forming a terminalof the cell.

DETAILED DESCRIPTION

The end cap assembly 10 (FIG. 1) of the invention may be applied toprimary or secondary (rechargeable) cells. In a preferred embodiment theend cap assembly 10 is insertable into the open end 95 of a typicallycylindrical casing 90 for the cell (FIG. 7). The cells contain apositive electode (cathode on discharge), a negative electrode (anode ondischarge), separator and electrolyte and positive and negative externalterminals in electrical communication with the positive and negativeelectrodes, respectively.

Referring now to FIG. 1 of the drawings, an end cap assembly 10 intendedfor insertion into the open end of a cell case comprises a thermallyactivatable current interrupter subassembly 38 and a pressure reliefsubassembly 48 integrated therein. Subassemblies 38 and 48 are separatedby a common support plate 60. Subassemblies 38 and 48 are held within acover 30 which defines the outer wall of the end cap assembly 10.Interrupter subassembly 38 is defined at its top end by a cup-shaped endcap plate 20 and at its bottom end by a contact plate 15 which is weldedto support plate 60. Cup shaped end cap plate 20 forms one of theexternal terminals of the cell. Support plate 60 separates chamber 68within thermal subassembly 38 from chamber 78 within pressure reliefsubassembly 48. Contact plate 15 is electrically connected to supportplate 60 which in turn is electrically connected to an electrode 88(anode or cathode) of the cell when end cap assembly 10 is applied to acell. A thermally responsive circuit interrupter mechanism (40,50) isprovided to complete the circuit between contact plate 15 and end cap20. If temperature within the cell exceeds a predetermined thresholdvalue the interrupter mechanism activates breaking electrical contactbetween end cap 20 and contact plate 15 thereby preventing current fromflowing through the cell.

The pressure relief subassembly 48 comprises a thin metallic diaphragm70 connected to a pressure resistant plate 80 which in turn iselectrically connected to a cell electrode 88 through conductive tab 87which is welded to plate 80. (Pressure resistant plate is electricallyconductive and of sufficient thickness that it does not substantiallydeform at elevated pressures at least up to about 600 psi (4.14×10⁶pascal.) If gas pressure within the cell builds up to exceed apredetermined threshold value diaphragm 70 bulges outwardly to breakelectrical contact with pressure resistant plate 80 thereby preventingcurrent from flowing to or from the cell. Pressure resistant plate 80and support plate 60 preferably also have perforations, 73 and 63,respectively, therein which helps to vent gas and relieve pressurebuildup within the cell.

In the preferred embodiment shown in FIG. 1 end cap assembly 10 may beused in a rechargeable cell, for example, a lithium-ion rechargeablecell. (A lithium-ion rechargeable cell is characterized by the transferof lithium ions from the negative electrode to the positive electrodeupon cell discharge and from the positive electrode to the negativeelectrode upon cell charging. It may typically have a positive electrodeof lithium cobalt oxide (Li_(x) CoO₂) or lithium manganese oxide ofspinel crystalline structure (Li_(x) Mn₂ O₄) and a carbon negativeelectrode. The negative electrode constitutes the anode of the cellduring discharge and the cathode during charging and the positiveelectrode constitutes the cathode of the cell during discharge and theanode during charging. The electrolyte for such cells may comprise alithium salt dissolved in a mixture of non-aqueous solvents. The saltmay be LiPF₆ and the solvents may advantageously include dimethylcarbonate (DMC), ethylene carbonate (EC), propylene carbonate (PC) andmixtures thereof. The present invention is applicable as well to otherrechargeable cells, for example, nickel metal hydride cells and nickelcadmium cells. End cap assembly 10 comprises an end cap terminal 20which is typically the positive terminal of the rechargeable cell, ametal support plate 60 which forms a support base under the cap plate20, and an insulator disk 35 between end cap 20 and support plate 60.Cap assembly 10 is advantageously also provided with a pressure reliefdiaphragm 70 below support plate 60 as shown in FIG. 1. Diaphragm 70 maybe welded to an underlying pressure resistant plate 80. This may beconveniently done by welding the base 72 of diaphragm 70 to a raisedportion 82 of underlying pressure resistant plate 80. Diaphragm 70should be of material that is electrically conductive and of minimalthickness of between about 0.1 and 0.5 millimeter, depending on thepressure at which the diaphragm is intended to actuate. The diaphragm 70may desirably be of aluminum. The diaphragm 70 is advantageously coinedso that it ruptures at a predetermined pressure. That is, the diaphragmsurface may be stamped or etched so that a portion of the surface is ofsmaller thickness than the remainder. One preferred diaphragm 70 for usein the present invention is coined to impose a semicircular or "C"shaped groove 70a in its surface. The shape of the groove advantageouslyis the same or similar to the shape of a major portion of the peripheraledge of diaphragm 70 and positioned advantageously in proximity to theperipheral edge. The particular pressure at which venting takes place iscontrollable-by varying parameters such as the depth, location or shapeof the groove as well as hardness of the material. When pressure becomesexcessive the diaphragm will rupture along the groove line. End cap 20and support plate 60 define a chamber 68 therebetween in which issituated a thermally activated current interrupter subassembly 38.Insulator disk 35 is formed of a peripheral base portion 35a and adownwardly sloping arm 35b extending therefrom. Arm 35b extends intochamber 68. Diaphragm 70 is designed to rupture when gas buildup withinthe cell reaches a predetermined threshold level. The region betweensupport plate 60 and diaphragm 70 forms a chamber 78 into which gasbuildup within the cell may vent upon rupture of diaphragm 70.

Current interrupter subassembly 38 comprises a thermally responsivebimetallic disk 40, a metallic contact plate 15 in electrical contactwith a resilient springlike member 50. As shown in FIGS. 1 and 5resilient member 50 may be formed of a single flexible member having anouter circular peripheral portion 50a from which a disk retainer tabportion 50c extends radially inward to generally hold bimetallic disk 40freely in place during any orientation of the cell while not restrictingits snap acting movement. This member can be welded at one point ofouter portion 50a to end cap plate 20 with a center contact portion 50bin contact with plate 15. Additionally, contact portion 50b can bedesigned with a reduced cross sectional area so that it can act as adisintegratable fuse link to protect against power surge conditions.Bimetallic disk 40 is positioned to freely engage sloping arms 35b ofinsulator disk 35 which arms act as the disk seat for disk 40.Bimetallic disk 40 also preferably includes a central aperture forreceiveing a raised contacting portion of metallic contact plate 15.Contact plate 15 is preferably welded to support plate 60 and provides asurface for resilient member 50 to rest as shown in FIG. 1. There is anelectrically insulating grommet 25 which extends over the peripheraledge of end cap 20 and along the bottom peripheral edge of diaphragm 70.Grommet 25 also abuts the outer edge of subassembly 38 as shown inFIG. 1. There may be a ring of metal 55 which is crimped over the topedge of grommet 25 and pressed against diaphragm 70 to seal the end capassembly interior components. The grommet 25 serves to electricallyinsulate the end cap 20 from the crimp ring 55 and also to form a sealbetween support plate 60 and crimp ring 55. The cover 30 of the end capassembly 10 may be formed from truncated cylindrical member shown bestin FIG. 5. In a completed cell assembly (FIG. 8) the outside surface ofcover 30 will come into contact with the inside surface of cell casing90. Support plate 60 provides a base for components of subassembly 38 torest and preferably is of bow shape to maintain active radialcompressive force against the inside surface of grommet 25. Supportplate 60 may be provided with perforations 63 in its surface to vent gasto upper chamber 68 when diaphragm 70 ruptures. Gas which passes intoupper chamber 68 will vent to the external environment through primaryvent holes 67 in end cap 20. The end cap assembly cover 30 is in contactwith the cell casing 90 which is in electrical contact with the oppositeterminal, typically the negative terminal in the case of a lithium-ionrechargeable cell. Thus, grommet 25 provides electrical insulationbetween end cap 20 and outer wall 30, that is, between the two terminalsof the cell thereby preventing shorting of the cell. There may be anadditional insulator ring, namely insulator standoff ring 42 between thetop portion of outer wall 30 and pressure plate 80 as illustrated inFIG. 1, also to assure that there is no shorting between the positiveand negative terminals of the cell.

Diaphragm 70 is preferably in the shape of a cup comprised of aluminumhaving a thickness advantageously of between about 3 and 10 mils. Atsuch thickness the weld between diaphragm base 72 and support plate 80breaks and the diaphragm base 72 bulges and separates from support plate80 (FIG. 3) when internal gas pressure within the cell rises to athreshold value of at least between about 100 psi and 200 psi (6.894×10⁵and 13.89×10⁵ pascal). (Such pressure buildup could occur for example ifthe cell were being charged at higher than recommended voltage or if thecell were shorted or misused.) However, if desired the thickness ofdiaphragm base 72 can be conveniently adjusted to bulge at otherpressure levels. The separation of diaphragm base 72 from plate 80breaks all electrical contact between the diaphragm 70 and plate 80.This separation also breaks the electrical pathway between end cap 20and the cell electrode 88 in contact with plate 80 so that current canno longer flow to or from the cell, in effect shutting down the cell.Even after the current path is broken if the pressure within the cellcontinues to rise for other reasons, for example, heating in an oven,the vent diaphragm 70 will also rupture preferably at a thresholdpressure of at least between about 250 and 400 psi (17.2×10⁵ and27.6×10⁵ pascal) to prevent cell explosion. In such extremecircumstances the rupture of vent diaphragm 70 allows gas from the cellinterior to vent through vent holes 73 (FIGS. 1 and 6) in pressureresistant plate 80 whereupon the gas enters lower chamber 78 (FIG. 1).The gas will then pass from lower chamber 78 to upper chamber 68 throughvent holes 63 in the support plate 60 (FIG. 1) and if needed vent holes(not shown) in insulator disk 35. Gas collected in the upper chamber 68will vent to the external environment through primary vent holes 67 inthe end cap plate 20.

The current interrupt features of the invention may be described withreference to FIGS. 1-3. It should be noted that in the specificembodiment shown therein one of the cell electrodes comes into contactwith plate 80 through tab 87 when the end cap assembly 10 is applied toa cell. During normal cell operation plate 80 in turn is electricallyconnected to end cap plate 20. In a lithium-ion cell the electrode 88 incontact with plate 80 may conveniently be the positive electrode. Thiselectrode will be insulated from the cell casing 90. The negativeelectrode (not shown) will be connected to the cell casing 90. Theembodiment of FIG. 1 shows the end cap assembly configuration beforecurrent is interrupted by either activation of the thermal currentinterrupter bimetallic disk 40 or activation of pressure reliefdiaphragm 70. In the specific embodiment illustrated in FIG. 1 plate 80is in electrical contact with diaphragm 70 and diaphragm 70 is inelectrical contact with support plate 60. Support plate 60 is inelectrical contact with contact plate 15 which is in electrical contactwith resilient member 50 which in turn is in electrical contact with endcap 20. In the integrated end cap design of the invention shown in FIG.1, electrical contact between the electrode 88 in contact with pressureplate 80 and end cap 20 may be interrupted in two ways. As abovedescribed if pressure builds up in the cell to a predeterminedthreshold, contact between diaphragm 70 and pressure plate 80 is brokenas diaphragm base 72 bulges away from pressure plate 80. Thisinterruption in the circuit prevents current from flowing to or from thecell. Alternatively, if the cell overheats the bimetallic disk 40 ofthermal interrupt subassembly 38 activates and in so doing pushesupwardly from insulator 35b thereby causing resilient member 50 todisengage from contact plate 15. This in effect severs the electricalpath between electrode tab 87 and end cap 20, thus preventing currentfrom flowing to or from the cell. It is an advantage of the invention toincorporate these two interrupt mechanisms within a single end capassembly 10 which is insertable into the open end of a cell case as asingle unit.

The bimetallic disk 40 is preferably not physically attached tounderlying insulator disk 35 but rather is free to move, that is itrests in free floating condition on disk arm 35b as shown in FIG. 1. Insuch design current does not pass through the bimetallic disk 40 at anytime regardless of whether the cell is charging or discharging. This isbecause disk 40 when inactivated is not in electrical contact withcontact plate 15. However, should the cell overheat beyond apredetermined threshold temperature, bimetallic disk 40 is designed tothe appropriate calibration such that it snaps or deforms (FIG. 2)causing it to push resilient member 50 away from contact plate 15thereby preventing current from flowing between the cell terminals. Thebimetallic disk 40 is calibrated so that it has a predetermined dishedshape which allows the disk to actuate when a given thresholdtemperature is reached. The free floating design of bimetallic disk 40on insulator disk arm 35b as above described does not permit current topass therethrough at any time regardless of whether the cell is chargingor discharging. This makes the calibration of disk 40 easier and moreaccurate, since there is no heating effect caused by current flowthrough bimetallic disk 40 (I² R heating). Bimetallic disk 40 mayconveniently comprise two layers of dissimilar metals having differentcoefficient of thermal expansion. The top layer of bimetallic disk 40(the layer closest to end cap 20) may be composed of a high thermalexpansion metal, preferably nickel-chromium-iron alloy and theunderlying or bottom layer may be composed of a low thermal expansionmetal, preferably nickel-iron alloy. In such embodiment disk 40 mayactivate when temperature rises to at least between about 60° to 750° C.causing disk 40 to deform sufficiently to push resilient member 50 awayfrom contact with contact plate 15. It is also possible to choose thehigh and low thermal expansion metal layers such that the disk 40 willnot reset except at a temperature below -200° C. which in mostapplications makes the device a single action thermostatic device.

Preferred materials for the above described components are described asfollows: End cap 20 is preferably of stainless steel or nickel platedsteel of between about 8 to 15 mil (0.2 and 0.375 mm) thickness toprovide adequate support, strength and corrosion resistance. The outerwall 30 of the end cap assembly 10 is also preferably of stainless steelor nickel plated steel having a thickness of between about 8 and 15 mil(0.2 and 0.375 mm. Pressure plate 80 is preferably of aluminum having athickness between about 10 and 20 mils (0.25 and 0.5 mm) which may bereduced at the center to between about 2 and 5 mils (0.05 and 0.125 mm)at the point of welded contact with diaphragm base 72. Insulatorstandoff ring 42 may be composed of a high temperature thermoplasticmaterial such as high temperature polyester for strength and durabilityavailable under the trade designation VALOX from General ElectricPlastics Company. Crimp ring 55 is preferably of stainless steel ornickel plated steel having a thickness between about 8 and 15 mils (0.2and 0.375 mm) for strength and corrosion resistance. Diaphragm 70 ispreferably of aluminum having a thickness of between about 3 and 10 mils(0.075 and 0.25 mm). At such thickness the diaphragm will break awayfrom its weld to pressure plate 80 when the internal gas pressureexceeds a threshold pressure between about 100 and 250 psi (6.89×10⁵ and17.2×10⁵ pascal). Should the internal gas pressure exceed a pressurebetween about 250 and 400 psi (17.2×10⁵ and 27.6×10⁵ pascal) diaphragm70 will rupture to provide additional relief from gas pressure buildup.The insulator disk 35 on which bimetallic disk 40 rests is preferably ofa material of high compressive strength and high thermal stability andlow mold shrinkage. A suitable material for disk 35 is a liquid crystalpolymer or the like of thickness between about 10 and 30 mils (0.25 and0.75 mm) available under the trade designation VECTRA from the CelaneseCo. Support plate 60 is preferably of stainless steel or nickel platedsteel to provide adequate strength and corrosion resistance at athickness of between about 10 and 30 mils (0.25 and 0.75 mm). Resilientmember 50 is advantageously formed of berylium-copper, nickel-copperalloy, stainless steel or the like which has good spring action andexcellent electrical conductivity. A suitable thickness for resilientmember 50 when formed of beryllium-copper or nickel-copper alloy isbetween about 3 and 8 mils (0.075 and 0.2 mm) to give sufficientstrength and current carrying capability. This material may be plated orinlayed with silver or gold at the contact region to provide lowerelectrical resistance in this area. Contact plate 15 is advantageouslyformed of cold rolled steel plated with a precious metal such as gold orsilver to lower contact resistance and improve reliability. It may alsobe formed of a nickel-copper clad alloy, stainless steel, ornickel-plated steel. Grommet 25 typically is made of polymeric matericalsuch as nylon or polypropylene. The seal around the end cap assemblycomponents should be hermetic in order that electrolyte, both in theform of liquid and vapor, is prevented from entering into the end capchambers or from leaving the cell.

After the end cap assembly 10 is completed it may be inserted into theopen end 95 of a cylindrical cell case 90 shown in FIG. 7. Thecircumferential edge of cell casing 90 at the open end thereof is weldedto the outer wall of cover 30 of end cap assembly 10 to provide ahermetically tight seal between end cap assembly 10 and the cell casing90. The radial pressure of the circumferential wall of crimp ring 55against grommet 25 and diaphragm 70 produces a hermetically tight sealaround the interior components of end cap assembly 10.

An alternative embodiment of the end cap design having both a pressurerelief mechanism and thermally activated current interrupt mechanismintegrated therein is shown as end cap assembly 110 in FIG. 4. Theembodiment of FIG. 4 is similar to that described above with respect toFIGS. 1-3 except that a bimetallic disk is not employed to activate thespringlike mechanism. Instead a thermal pellet 175 is provided to hold aresilient springlike member 150 in electrical contact with contact plate115. Contact plate 115 in turn is in electrical contact with end capplate 20. Resilient member 150 may comprise an elongated metallic arm150a which is welded at one end to support plate 60. Support plate 60 isin electrical contact with diaphragm 70 which in turn is welded to araised portion 82 of underlying pressure resistant plate 80. Anelectrode tab 87 is in electrical contact with plate 80. Resilientmember 150 preferably terminates at its opposite end in a cup or convexshaped portion 150b which contacts contact plate 115. There is anelectrical insulator disk 120 over the peripheral edge 60a of supportplate 60 to prevent direct contact between support plate 60 and contactplate 115. Thus, there will be electrical contact between support plate60 and end cap 20 as long as resilient member 150 is held pressedagainst contact plate 115. Support plate 60 in turn is in electricalcontact with aluminum diaphragm 70 which is in contact with plate 80 anda cell electrode 88 through tab 87 when the end cap assembly 110 isapplied to a cell. (End cap assembly 110 may be applied to a cell byinserting it into the open end of a cylindrical casing 90 in the samemanner as above described with reference to the embodiment shown in FIG.1.) Therefore, as resilient member 150 is held pressed against contactplate 115 by thermal pellet 175, there is electrical contact between acell electrode 88 (through tab 87) and end cap plate 20 permittingnormal cell operation. If the cell overheats beyond a predeterminedthreshold temperature pellet 175 melts thereby removing support forresilient member 150. Melting of pellet 175 causes resilient member 150to snap downwardly and break electrical contact with contact plate 115.This in effect severs the electrical pathway between the electrode tab87 and end cap 20 thus preventing current from flowing to or from thecell. If the internal gas pressure within the cell exceeds apredetermined value diaphragm 70 will rupture thereby severingelectrical contact between plate 80 and diaphragm 70 and also allows gasto escape to the external environment through vent holes 63 and 67 insupport plate 60 and end cap 20, respectively.

Preferred materials for the end cap 20, support plate 60, contact plate115 and aluminum diaphragm 70 reference in the embodiment shown in FIG.4 may be the same as described for the corresponding elements having thesame reference numerals shown in FIGS. 1-3. Contact plate 115 ispreferably formed of stainless steel or nickel plated cold rolled steelplated with silver or gold to lower its contact resistance. Theinsulator disk 120 shown in FIG. 4 is preferably made of a hightemperature thermoplastic material having excellent dielectricproperties. A preferred material for disk 120 may a polyimide availableunder the trade designation KAPTON from E. I. DuPont Co. or hightemperature polyester available under the trade designation VALOX fromGeneral Electric Plastics Co. Resilient member 150 may advantageously beformed of beryllium-copper alloy of thickness between about 5 and 10mils (0.125 and 0.25 mm) to provide good conductivity when in contactwith plate 115 and reliable spring action when the pressure of pellet175 against it is removed. Additionally, the resilient arm 150 may beplated with silver or gold to increase its conductivity. The thermalpellet 175 is advantageously formed of a polymer having a relatively lowmelting point, e.g., between about 65° C. and 100° C. but yet excellentcompressive strength to keep the resilient arm 150 in place duringnormal cell operation. A suitable material for thermal pellet 175 havingsuch properties is a polyethylene wax available under the tradedesignation POLYWAX from Petrolyte Co. A thermal pellet 175 of suchpolyethylene wax melts within a desirable temperature range of betweenabout 75° C. and 80° C.

An exploded view of the end cap assembly 10 of FIG. 1 is shown in FIG.5. The end cap assembly 10 may be made by assembling the componentsshown in FIG. 5 in the following order: A preassembly is formedcomprising components 20, 50, 40, 35, 15, 60, 70, 25, and 55. This isconveniently accomplished by first inserting plastic grommet 25 intocrimp ring 55, then inserting vent diaphragm 70 into grommet 25 and theninserting support plate 60 with contact plate 15 welded thereto intovent diaphragm 70. Thereupon insulator disc 35 is placed around contactplate 15 and bimetallic disk 40 is placed to rest on downwardly slopingarm 35b of insulator disk 35. Bimetallic disk 40 is not bonded toinsulator disk 35 but rests thereon in a free floating condition, withthe insulator disk helping to act as positionaing means for thebimetallic disk. The top surface of the outer end of resilientspringlike member 50 is welded to the circumferential edge of end cap20. The end cap 20 with resilient member 50 welded thereto is thenplaced over insulator disk 35 so that the raised central portion ofcontact plate 15 contacts the interior end of resilient member 50 andthe bottom surface of the outer end of resilient member 50 contacts thecircumferential edge of insulator disk 35. Thus, the outer end ofresilient member 50 is wedged between end cap 20 and insulator disk 35and the opposite or inner end of member 50 is in contact with contactplate 15. Then, ring 55 is mechanically crimped over the top edge ofgrommet 25 to hold the top end of grommet 25 tightly pressed against thecircumferential edge of end cap 20. This crimping is accomplished byapplying mechanical force along the centroidal (vertical) axis of ring55. Then in a second crimping step mechanical pressure is appliedradially to the walls of crimp ring 55, thereby completing assembly ofthe preassembly. The radial crimping serves to keep the preassemblyinternal components tightly and hermetically sealed within the ring 55.The preassembly is then inserted into metallic cover 30 so that thebottom surface of crimp ring 55 rests against the bottom inside edge ofcover 30. Thereupon the bottom surface of crimp ring 55 is welded to thebottom inside surface of cover 30. Pressure plate 80 is then snappedinto the bottom of insulator standoff ring 42 and the standoff ring 42with pressure plate 80 attached thereto is then placed against theoutside bottom surface of cover 30 so that the raised central portion ofpressure plate 80 contacts vent diaphragm 70. This point of contactbetween pressure plate 80 and diaphragm 70 is then spot welded thuscompleting construction of end cap assembly 10. The end cap assembly 10may be applied to a cell, for example, by inserting it into the open endof the cylindrical casing 90 of a cell as shown in FIG. 7 and weldingthe outer surface of cover 30 to the inside surface of the cylindricalcasing 90 at the open end 95 thereof. This results in cell 100 shown inFIG. 8 with end cap assembly 10 being tightly sealed within thecylindrical casing 90 and the end cap plate 20 forming a terminal of thecell.

While this invention has been described in terms of certain preferredembodiments, the invention is not to be limited to the specificembodiments but rather is defined by the claims and equivalents thereof.

What is claimed is:
 1. An end cap assembly for application to anelectrochemical cell having a positive and a negative terminal and apair of internal electrodes (anode and cathode), said end cap assemblycomprising a housing and an end cap plate, said plate functional as acell terminal, said end cap assembly having an electrically conductivepathway therethrough permitting the end cap plate to be electricallyconnected to a cell electrode when said end cap assembly is applied to acell, said end cap assembly further comprising a) thermally responsivemeans for preventing current from flowing through said electricalpathway, wherein said thermally responsive means is activatable when thetemperature within said end cap assembly reaches a predetermined levelcausing a break in said electrical pathway and b) pressure responsivemeans for preventing current from flowing through said electricalpathway, wherein said pressure responsive means is activatable when gaspressure applied to at least a portion of said electrical pathwayreaches a predetermined level causing a break therein.
 2. The end capassembly of claim 1 wherein said end cap assembly is applied to anelectrochemical cell by inserting it into the open end of a casing ofsaid cell and welding the end cap assembly to the casing.
 3. The end capassembly of claim 1 wherein said thermally responsive means comprises abimetallic member and a resilient electrically conductive member, theresilient member forming a portion of said electrical pathway, whereinwhen the temperature within said assembly reaches a predetermined levelthe bimetallic member deforms thereby pushing against said resilientmetallic member causing a break in said electrical pathway.
 4. The endcap assembly of claim 3 wherein the conductive member has a portion ofreduced cross sectional area to act as a disintegrable fuse link toprotect against power surge conditions.
 5. The end cap assembly of claim3 wherein the bimetallic member is movably positioned to engage a diskseat on a surface of an electrically insulating member within said endcap assembly.
 6. The end cap assembly of claim 5 wherein a portion ofsaid resilient conductive member is sandwiched between a portion of saidend cap plate and a portion of said electrically insulating member. 7.The end cap assembly of claim 3 wherein said resilient conductive memberincludes a retainer means for limiting movement of said bimetallicmember while not restricting its deforming action when the temperaturewithin said assembly reaches a predetermined level.
 8. The end capassembly of claim 3 wherein said pressure responsive means comprises adiaphragm plate adapted to be electrically connected to a cell electrodewhen said end cap assembly is applied to an electrochemical cell,wherein said diaphragm plate deforms to cause a break in said electricalpathway thereby severing electrical connection between said end capplate and said electrode when said end cap assembly is applied to a celland the cell internal gas pressure reaches a first predetermined level.9. The end cap assembly of claim 8 further comprising a support plateplaced across the interior width of the end cap assembly, said supportplate dividing the interior of the end cap assembly into an upperchamber and a lower chamber, the upper chamber being closer to the endcap plate than the lower chamber.
 10. The end cap assembly of claim 9further comprising an electrically conductive pressure resistant platelocated at the end of the end cap assembly opposite said end cap plate,wherein said diaphragm plate is welded to the electrically conductivepressure resistant plate, said pressure resistant plate having at leastone aperture therethrough, wherein a portion of said pressure resistantplate and a portion of said diaphragm forms at least part of saidelectrically conductive pathway.
 11. The end cap assembly of claim 10wherein said diaphragm plate ruptures when said end cap assembly isapplied to a cell and gas pressure within the cell reaches a secondpredetermined level greater than said first predetermined level,whereupon gas from the interior of the cell passes through said ruptureddiaphragm plate and into said lower chamber.
 12. The end cap assembly ofclaim 10 wherein said diaphragm plate has a grooved path therein ofreduced thickness.
 13. The end cap assembly of claim 12 wherein saidgrooved path conforms generally to a major portion of the outerperipheral shape of the diaphragm plate wherein said grooved pathprovides a rupture path for said diaphragm.
 14. The end cap assembly ofclaim 13 wherein said grooved path is of substantially "C" shape. 15.The end cap assembly of claim 11 wherein the support plate has at leastone aperture therethrough so that gas from the interior of the cellwhich accumulates in the lower chamber may pass through said supportplate aperture and into the upper chamber.
 16. The end cap assembly ofclaim 15 wherein the end cap plate has at least one aperturetherethrough so that gas which passes into said upper chamber may passthrough said end cap aperture to the external environment.
 17. The endcap assembly of claim 9 further comprising an electrically conductivecontact plate welded to a surface of said support plate, wherein aportion of said resilient conductive member contacts said contact plateand wherein at least a portion of said contact plate and said resilientconductive member form a part of said electrically conductive pathway.18. The end cap assembly of claim 9 wherein the peripheral edge of saidsupport plate contacts the peripheral edge of said diaphragm plate andthe diaphragm plate is electrically connectable to a cell electrode whenthe end cap assembly is applied to a cell.
 19. The end cap assembly ofclaim 17 wherein when said bimetallic member reaches a predeterminedtemperature it deforms causing the resilient conductive member to severits electrical connection with said contact plate.
 20. The end capassembly of claim 11 further comprising an electrically insulatinggrommet in contact with the peripheral edge of the end cap plate and theperipheral edge of the diaphragm plate, said end cap assembly furthercomprising a metallic crimping member mechanically crimped around saidgrommet to hold said diaphragm plate and said support plate undermechanical compression.
 21. The end cap assembly of claim 20 furthercomprising a metallic cover contacting a portion of said crimping memberand protecting the outside surface of said crimping member.
 22. The endcap assembly of claim 17 wherein the end cap assembly is applied to acell by inserting it into the open end of a cylindrical casing for thecell and welding the outside surface of said cover to the inside surfaceof said casing, whereupon the end cap assembly becomes tightly sealedwithin the cylindrical case with the end cap plate comprising a terminalof the cell being exposed to the external environment.
 23. The end capassembly of claim 1 wherein the thermally responsive means comprises aresilient conductive member in electrical contact with said end capplate, and a meltable mass of material holding said resilient conductivemember in electrical connection between said end cap plate and anotherconductive portion of the end cap assembly, said other conductiveportion adapted to be electrically connected to a cell electrode whenthe end-cap assembly is applied to a cell thereby providing anelectrical connection between said end cap plate and said electrodeduring cell operation, wherein when the cell temperature reaches apredetermined level said mass of material melts thereby causing movementin said resilient metallic member to sever the electrical connectionbetween said end cap plate and said cell electrode thereby preventingoperation of the cell.
 24. In an electrochemical cell of the type formedby an end cap assembly inserted into an open ended cylindrical case forthe cell, said cell further having a positive and a negative terminaland a pair of internal electrodes (anode and cathode), wherein said endcap assembly has a housing and an exposed end cap plate, said end capplate functional as a cell terminal, the improvement comprising said endcap plate being electrically connected to one of said electrodes throughan electrically conductive pathway within said end cap assembly, saidend cap assembly further comprising a) thermally responsive means forpreventing current from flowing through the cell, wherein said thermallyresponsive means is activatable when the temperature within the cellreaches a predetermined level causing a break in said electrical pathwaybetween said end cap plate and said electrode thereby preventing currentfrom flowing through the cell and b) pressure responsive means forcausing a break in said electrical pathway between said end cap plateand said electrode when the internal gas pressure within the cellreaches a predetermined level thereby preventing current from-flowingthrough the cell.
 25. The electrochemical cell of claim 24 wherein saidthermally responsive means for preventing current flow through the cellcomprises a bimetallic member and a resilient electrically conductivemember, the resilient member being in electrical contact with said endcap plate and a cell electrode, thereby providing an electrical pathwaytherebetween, wherein when the cell temperature reaches a predeterminedlevel the bimetallic member deforms and pushes against said resilientmember causing said resilient member to sever said electrical pathwaybetween the end cap plate and said cell electrode thereby preventingoperation of the cell.
 26. The electrochemical cell of claim 25 whereinthe bimetallic member is movably positioned to engage a disk seat on asurface of an electrically insulating member within said end capassembly.
 27. The end cap assembly of claim 25 wherein the resilientconductive member has a portion of reduced cross sectional area to actas a disintegrable fuse link to protect against power surge conditions.28. The electrochemical cell of claim 26 wherein a portion of saidresilient conductive member is sandwiched between a portion of said endcap plate and a portion of said electrically insulating member.
 29. Theelectrochemical cell of claim 24 wherein said pressure responsive meanscomprises a diaphragm plate adapted to be placed in electrical contactwith a cell electrode when said end cap assembly is applied to anelectrochemical cell, wherein said diaphragm plate deforms to cause abreak in said electrical pathway thereby severing electrical connectionbetween said end cap plate and said electrode when the cell internal gaspressure reaches a first predetermined level.
 30. The electrochemicalcell of claim 29 further comprising a support plate placed across theinterior width of the end cap assembly, said support plate dividing theinterior of the end cap assembly into an upper chamber and a lowerchamber, the upper chamber being closer to the end cap plate than thelower chamber.
 31. The electrochemical cell of claim 30 furthercomprising an electrically conductive pressure resistant plate locatedat the end of the end cap assembly opposite said end cap plate, whereinsaid diaphragm plate is welded t o an electrically conductive pressureresistant plate, said pressure resistant plate having a t least oneaperture therethrough, wherein at least a portion of said pressureresistant plate and said diaphragm plate forms a part of said electricalpathway.
 32. The electrochemical cell of claim 31 wherein said diaphragmplate ruptures when said end cap assembly is applied to a cell and gaspressure within the cell reaches a second predetermined level greaterthan said first predetermined level whereupon gas from the interior ofthe cell passes through said ruptured diaphragm plate and into saidlower chamber.
 33. The end cap assembly of claim 31 wherein saiddiaphragm plate has a grooved path therein of reduced thickness.
 34. Theend cap assembly of claim 33 wherein said grooved path conformsgenerally to a major portion of the outer peripheral shape of thediaphragm plate wherein said grooved path provides a rupture path forsaid diaphragm.
 35. The end cap assembly of claim 34 wherein saidgrooved path is of substantially "C" shape.
 36. The electrochemical cellof claim 32 wherein the support plate has at least one aperturetherethrough so that gas from the interior of the cell which accumulatesin the lower chamber may pass through said support plate aperture andinto the upper chamber.
 37. The electrochemical cell of claim 36 whereinthe end cap plate has at least one aperture therethrough so that gaswhich passes into said upper chamber may pass through said end capaperture to the external environment.
 38. The electrochemical cell ofclaim 30 further comprising an electrically conductive contact platewelded to a surface of said support plate, wherein a portion of saidresilient member contacts said contact plate and said support plate iselectrically connectable to an electrode of the cell when the end capassembly is applied to a cell.
 39. The electrochemical cell of claim 30wherein the peripheral edge of said support plate contacts theperipheral edge of said diaphragm plate and the diaphragm plate iselectrically connectable to a cell electrode when the end cap assemblyis applied to a cell.
 40. The electrochemical cell of claim 38 whereinwhen the cell temperature reaches a predetermined temperature saidbimetallic member deforms causing the metallic resilient member to severits electrical connection with said contact plate.
 41. Theelectrochemical cell of claim 37 further comprising an electricallyinsulating grommet in contact with the peripheral edge of the end capplate and the peripheral edge of the diaphragm plate, said end capassembly further comprising a metallic crimping member mechanicallycrimped around said grommet to hold said diaphragm plate and saidsupport plate under mechanical compression.
 42. The electrochemical cellof claim 41 further comprising a metallic cover contacting a portion ofsaid crimping member and protecting the outside surface of said crimpingmember.
 43. The electrochemical cell of claim 42 wherein the end capassembly is applied to a cell by inserting it into the open end of acylindrical casing for the cell and welding the outside surface of saidcover to the inside surface of said casing, whereupon the end capassembly becomes tightly sealed within the cylindrical case with the endcap plate comprising a terminal of the cell being exposed to theexternal environment.
 44. The electrochemical cell of claim 24 whereinthe thermally responsive means comprises a resilient conductive memberin electrical contact with said end cap plate, and a meltable mass ofmaterial holding said resilient member in electrical connection betweensaid end cap plate and another conductive portion of the end capassembly, said other conductive portion adapted to be electricallyconnected to a cell electrode when the end cap assembly is applied to acell thereby providing an electrical connection between said end capplate and said electrode during cell operation, wherein when the celltemperature reaches a predetermined level said mass of material meltsthereby causing movement in said resilient member to sever theelectrical connection between said end cap plate and said cell electrodethereby preventing operation of the cell.
 45. A current interrupter forproviding protection to a device being supplied with electrical currentcomprising a housing member having an electrically conductive pathwaytherethrough in electrical connection with said device, a thermallyresponsive means contained within said housing member for preventingcurrent from flowing through said electrical pathway, wherein saidthermally responsive means is activated when the temperature within saidhousing reaches a predetermined level causing a break in said electricalpathway, and a pressure responsive means contained within said housingseparate from said thermally responsive means for preventing currentfrom flowing through said electrical pathway, wherein said pressureresponsive means is activated when fluid pressure reaches apredetermined level causing a break in said electrical pathway.
 46. Thecurrent interrupter of claim 45 wherein said thermally responsive meanscomprises a bimetallic member and a resilient electrically conductivemember, the resilient member forming a portion of said electricalpathway, wherein when said bimetallic member reaches a predeterminedlevel, it snaps thereby causing said resilient member to move causing abreak in said electrical pathway.
 47. The current interrupter of claim46 wherein said resilient electrically conductive member has a portionof reduced cross-sectional area to act as fuse link to protect againstpower surge conditions.
 48. The current interrupter of claim 45 whereinsaid pressure responsive means comprises an electrically conductivediaphragm member adapted to be electrically connected to said electricalpathway wherein when said pressure on said diaphragm member reaches afirst predetermined level said diaphragm member deforms to cause a breakin said electrical pathway.
 49. The current interrupter of claim 48wherein said diaphragm m ember ruptures when said fluid pressure uponsaid diaphragm member reaches a second predetermined level higher thansaid first predetermined level thereby allowing fluid to pass throughsaid diaphragm member.
 50. The current interrupter of claim 49 whereinsaid diaphragm member has a grooved path of reduced material thicknesstherein, wherein said grooved path acts as the rupture point when thefluid pressure on said diaghragm member reaches said secondpredetermined level.
 51. The current interrupter of claim 50 whereinsaid grooved path conforms generally to a major portion of the outerperipheral shape of the diaphragm element.